Dick was said to be 20 and 22 in a couple issues the late 1980s. It's only later in the post-COIE-era that the de-aging started for him (definitely by Nigthwing Year One, which said he was 21, which made no sense at all). Even before Flashpoint, Dick's age when starting as Robin has been all over the map - from 8 to 13. I'm pretty sure both 10 and 13 were used in the 1986-1990 era, but wouldn't swear to it. In this new timeline, he premieres as Robin the same year as Wally does as Flash. Wally was originally a 9th grader, but I know later they pushed him back to 13, and he may even have been made younger at some point - no idea. Since this version has the Teen Titans debut 3 years after Robin and Kid Flash, 10 is a minimum age for Dick donning the costume, I think, with 12 or 13 being more likely. New Teen Titans are 6 years after Robin debuts, so he's probably 13, if they want to keep that happening when Dick was 19. No guarantees, though.